It is a standard practice to form the wall of a steel-smelting furnace of a plurality of generally parallel tubes. The ends of these tubes are connected together by means of 180.degree. reverse bends and a coolant is pumped through them to prevent the tubes from heating so much that they soften.
In order to make the furnace wall gastight, it is standard practice to provide a second sheet-steel wall adjacent such a tube wall. The resultant structure is therefore both cooled and gastight.
A disadvantage of this type of arrangement is that the tubes are frequently damaged. This can happen during charging of the furnace with scrap. Making the tubes heavier to resist such damage merely increases their heat capacity and makes cooling them an even more expensive operation.